The present invention relates to a cathode for a high frequency electronic tube and more particularly to a direct heating thermo-electronic emission cathode.
In high frequency electronic tubes of the triode, tetrode or pentode type, which comprise a cathode, an anode and one, two or three grids, it is advantageous to form the grids from pyrolytic graphite, a material known for its mechanical and thermal qualities.
However, in these same tubes the cathodes are generally made from tungsten wires or thoriated tungsten wires for reasons of thermo-electronic emissivity, the operating temperature is then between 1900.degree. and 2000.degree. K. There then arise, during operation, mechanical problems because of the difference in thermal behaviour between the materials, which problems are imperfectly solved by means of expensive mechanical fittings. It has been proposed to avoid the thermo-mechanical problems inside the tube while ensuring good thermo-electronic emissivity by introducing direct heating from a pyrolytic graphite support and by depositing on the surface of the graphite a material emitting at a lower temperature than tungsten or thoriated tungsten such as lanthanum hexaboride LaB.sub.6 for example. Such a structure allows electronic emission to be obtained at a temperature between 1400.degree. and 1500.degree. C. However, a drawback of emissive materials such as lanthanum hexaboride is that of their high chemical activity with respect to hot graphite, which may lead to the destruction of the cathode. For this reason it is then necessary to insert an intermediate layer between the graphite and the lanthanum hexaboride forming a diffusion barrier between these two materials.